1. Field of the Invention
This invention relates to ophthalmic lenses and their production.
2. Description of the Prior Art
It is well known in the prescription of ophthalmic lenses that, in cases of anisometropia where the patients's eyes require different degrees of focal correction, there is a danger of producing the effect of double vision. This occurs when an object is viewed through the upper or lower parts of the unequal lenses, above or below the optical centers of the lenses, where the stronger lens will distort the vertical position of the object to a greater degree than a weaker lens. This disadvantage can be minimized by producing one or both lenses with upper and lower lens surfaces having different optical centers. In this way, the prismatic differences between right and left-hand lenses can be reduced, providing upper and lower angles of view with minimum displacement, in the upper and lower regions of the lenses respectively. However, such lenses have the disadvantage that they are traversed by unsightly horizontal lines where the upper and lower regions of the lenses meet, and that, at least when produced in glass, extensive accurate grinding by the so-called "slab-off" technique is involved which renders the finished lenses very expensive and not readily available as few people have the skill and capacity to make them. Moreover, in the case of deep minus lenses glass blanks of sufficient thickness for "slab-off" grinding are not readily available.
The presence of the horizontal lines in such lenses having two optical centers, hereinafter referred to as bicentric lenses, renders the lenses cosmetically unacceptable, since patients requiring right and left lenses of unequal power are often already conscious of their apparent uneqal eye size, and the presence in addition of unsightly horizontal lines in their lenses is a further embarrassment. In addition, the known bicentric lenses have the disadvantage that the sharp horizontal dividing line which separates the upper and lower sufaces of the lens has itself the effect of causing interference and double vision.